From the said research report, it is known to transmit digitally coded items of auxiliary information in an AM (amplitude modulated) radio broadcast signal by phase modulation and demodulation of the AM radio broadcast signal carrier. In order not to interfere with the programme signal of the phase modulated AM carrier, a value of .+-.22.5.degree. was selected by the British Broadcasting Corporation as the maximum permissible phase displacement which corresponded to a transmission capacity of 25 bit/s for the items of auxiliary information. This particularly small channel capacity requires an error protection which is matched as optimally as possible to the properties of the AM radio broadcast transmission channel in the sense of a low redundancy. To this end, the data stream is subdivided into blocks each having a length of 50 bits wherein each block consists of a synchronisation bit, an information word of 36 bits and a check word of 13 bits. An optimal error correcting code with which an error burst up to a length of 6 bits can be corrected is used for the block coding. The selected code is cyclical which is shortened to the block length of 50 bits. The synchronisation bit is placed in front of the block at the transmitting end in order to be able to recognise the start of the block with certainty but due to this the cyclical character of the block code is destroyed. A 4 bit wide block application code is transmitted at the beginning of the information word directly behind the synchronisation bit in order to identify the content of the 32 bit long information word. A special information coding in association with the block utilisation code 0000 is provided within the information word just for information regarding the time and the date.
As opposed to this, the object of the invention consists in reducing the redundancy in an auxiliary information signal of the type mentioned hereinabove as well as in simplifying the allocation and thus the recognition of different items of information.